The present invention relates generally to radar apparatus commonly referred to as repeaters used for the purpose of deceiving enemy radar as to the range and angle of a target detected thereby. More specifically, the present invention is concerned with detecting dual scan threat radar signals and generating in response thereto separate deception signals for transmission back to each antenna of the dual scan tracking radar causing it to detect an inaccurate target position.
Radar systems currently in use are typically of the conical scan or dual scan types. The conical scan (CS) radar involves the use of the single antenna system which is either fixed in position or sweeps over a selected path locking onto a target when detected. The dual scan or track-while-scan (TWS) radar typically incorporates two interrelated antenna systems in which one of the antenna is energized at a first frequency and caused to sweep in the horizontal direction while the other antenna is energized at a second frequency and caused to sweep in the vertical direction. Whereas a single channel deception apparatus may be appropriate for use in jamming conical scan systems, such apparatus will only have partial effectiveness against the dual scan type of system since each scan antenna of the dual scan system can independently provide usable range and angle information. In order to provide maximum deception capability a dual channel jamming system is thus clearly more desirable than the single channel system for use against dual scan systems.
In addition, power output and jamming effectiveness considerations make dual jamming of dual beam radars desirable. One of the primary difficulties involved in using a single channel jammer to jam a dual channel radar is that traveling-wave tubes (TWT's) of the type used in jamming systems, when operating in the saturation region, have several characteristics that reduce their effectiveness when amplifying two RF frequencies simultaneously. One of these is called the capture effect, in which two frequencies at different power levels will receive different amounts of gain while passing through the traveling-wave tube. The resultant effect is that the signals emerge from the tube with power levels which are more widely separated than they were upon entrance. If the two different power levels are from the main lobe and sidelobe of the two radar beams, the main lobe signal will receive more gain than a sidelobe signal, thus tending to enhance the target rather than obscure it.
Another problem encountered in the two frequency operation of a TWT is intermodulation distortion, which tends to mix the two RF frequencies and direct power into various harmonics spread through the spectrum at frequencies separated by the original frequency separation. This has the obvious effect of reducing the power available for the input signals, thus reducing the power output on the proper frequencies. Extreme cases of this phenomenon have been noted where little or no power emerges from the TWT on the frequencies that were put into it.
A third result of jamming a dual channel radar with a single channel jammer is that of cross modulation within the jammer itself. Jamming modulation techniques generally depend in some way on detecting the passage of the main lobe and either down-modulating the jammer output during main lobe passage or turning the jammer on fully just after the peak of the main lobe passage. Since electrical signals corresponding to the two beams are in a single channel jammer simultaneously, the jammer inadvertently modulates both channels of the radar when the intent is to modulate just one. The result is to create in the threat receiver a moving dark gap which periodically uncovers the real target. In the case of main lobe offset jamming, a false target is presented which moves across the radar screen at the beat rate of the two beams, but vanishes when the two main lobes cross.
These technical difficulties thus lead to the conclusion that separating the radar beams in the jamming system is highly desirable. In addition, beam separation makes available the unobscured individual scans of the radar for observation and manual modulation. It is also worthy of note that jamming is generally more effective when both channels of the radar are jammed instead of just one. This is because the tracking function must use the square root of the sum of the squares of the tracking errors in the three coordinates: azimuth, elevation, and range. Obviously, by increasing the tracking error in two of the three coordinates, the total tracking error will be larger than if tracking error is increased in only one of the three.
Another observation favoring a dual channel system is the apparent double pulsing capability of certain types of enemy radar wherein the pulses from the two RF channels may be separated in time. A single jamming unit typically has a recovery time approaching 1 microsecond after pulsing, during which the jammer cannot respond to another pulse. However, in a dual channel jammer of the type provided by the present invention, the pulse from each radar channel passes through a separate jammer unit and is treated separately, thus circumventing the problem of responding immediately to another pulse.
The following prior art patents are of possible interest: Byrne U.S. Pat. No. 2,658,992, Purington U.S. Pat. No. 2,676,317, Henrici et al U.S. Pat. No. 2,862,204, Pettit U.S. Pat. No. 2,989,744, Goldmark U.S. Pat. No. 3,142,060, Barney et al U.S. Pat. No. 3,225,300, Harpster U.S. Pat. No. 3,258,771 and Tolles et al U.S. Pat. No. 3,504,366.